Omar Kilani <omar(dot)kilani(at)gmail(dot)com> writes:
> This is a very old database (2004) that has moved forward via pg_upgrade. I
> think we did a pg_dump and pg_restore every time we hit some major
> incompatibility like float vs integer date times.
If it's that old, it's likely also survived multiple OS upgrades.
It seems clear that this index has been corrupt for awhile, and
I'm wondering whether the corruption was brought on by an OS
locale change. There's useful info at
https://wiki.postgresql.org/wiki/Locale_data_changes
regards, tom lane